Infrared spectroscopy had its origin when William Herschel discovered optical radiation beyond the red portion of the visible spectrum, which had been discovered by Isaac Newton. Since the 19th century, the interaction between infrared radiation and different substances had been studied by scholars in the fields of both physics and chemistry. It has since been found that different materials absorb different portions of the infrared spectrum and these absorption features can be used to detect and identify chemical species. The first mass-produced infrared spectrometer was not built until World War II, when the US Office of Rubber Reserve identified the use of infrared spectroscopy as an effective means for measuring the compositions of the synthetic rubber compounds, and demanded the development of infrared spectrometers capable of operation from 1 μm and beyond. Since 1950, infrared spectrometers have gained wide acceptance by both the scientific and engineering communities. The first generation of these spectrometers utilized the optical dispersion of infrared radiation by rock salt materials such as sodium chloride (NaCl). Later these hygroscopic, or moisture-sensitive, rock salt prisms were replaced by diffractive gratings made with glass substrates. In general, these early infrared spectrometers weighed over 200 pounds, and were packaged in a space larger than 7 to 8 cubic feet in volume.
In addition to the dispersion-based infrared spectrometers, in late 1960s, with the advancements in lasers, computers and data storage devices, a new type of infrared spectrometer based on the principle of optical interferometry, was developed. Due to the computation involved in converting the interferometric measurements to optical spectra by the mathematical calculation of Fourier Transformation, these infrared instruments are also known in the infrared spectroscopy art as FTIR. Despite the fact that these FTIR spectrometers have provided highly improved sensitivity and spectral resolution compared to the dispersive infrared spectrometers, the interferometric nature of these instruments requires that ultimate environmental control be observed during their operation. This is because the interferometric measurements require the overlapping of two optical beams within the distance of half a wavelength, i.e., a precision in the micrometer range. In other words, any minute changes in the temperature or any small amount of vibration would result in failure to obtain useable spectra from such an FTIR spectrometer. Due to the incorporation of the optical interferometer and all the related feedback and control electronics, an FTIR usually weighs from 150 to 400 pounds and occupies a volume of 3 to 10 cubic feet.
For many modern industrial, security and military tasks, the abilities to detect the infrared spectral signatures of chemicals on the surface or in the atmosphere is highly valuable. These types of chemical information often can be used to determine the quality of a production process, or to evaluate the danger to approach or enter an area. However, due to the limitations in size, weight and ruggedness, infrared spectrometers have not been widely used as a first-line detection or monitoring tools for these applications, but rather as an off-line or laboratory validation tool.
To alleviate the practical limitations mentioned above, it would be desirable to develop a sensitive, light-weight, rugged and compact infrared spectrometer capable of detecting and/or identifying different chemical species under hostile service conditions. To achieve these goals, a compact optical spectrometer design is needed to cover a wide range of the infrared wavelengths, especially one that falls into the so-called “infrared fingerprint” (7–14 μm) region. This is because the infrared absorption features in the fingerprint region will provide the most distinctive chemical evidence that can be used to identify different substances.
While it could be straightforward to use the principles behind an FTIR to build smaller spectrometers that cover a wide range of the infrared spectrum, the low resistance to environmental changes may not meet the ruggedness requirements for field applications.
Another consideration related to the economics of the instrument is the ease of assembly. Inside all the infrared spectrometers today, the majority of the optical components found are made of glass materials. Whether it is the grating in a dispersive instrument or the mirrors within an FTIR, these glass-based components are often glued or clamped to mounts before they can be secured and then aligned. The accuracy of the gluing process, the possible shrinkage of the glue or epoxy and the chipping of the glass edges, all place uncertainties on the precision of the final alignment. As a result, these instruments need frequent maintenance or alignment after leaving the assembly line. Even during the assembly process, highly skilled opticians are required for proper alignment of all of these optical components, making the production more costly. The present invention provides a solution to these and other shortcomings found in the prior art.